Fine tuning an 850, part 2

[Suzuki_Don]

The K&Ns arrived today and I had my toolbox opened before the mailman even left
The filters are on and I raised the needles a notch. Lost one needle E clip in the process but had a spare

Now need to find a bit of time and the rain to stop for some road testing.

Will keep you posted.

Good luck Ivor, hope it works out for you this time. We all desire smooth, rapid power delivery.

[psyguy]

update well overdue

bike's been off the road for a while for (ahem ahem ) more cosmetic enhancements... but is now back together so i was able to test ride it after i played with the carbs:

lifting the needles by a notch took almost all the popping out!

though, something's not quite right now just off the idle... but i guess that at 4 turns out the pilot screws are too rich so i'll play again with the "highest rpm method" to set them correctly

(previously, i turned the pilot screws down but the popping increased so i turned them out again to decrease the popping. obviously, i was working with the wrong carb circuit - pilot instead of needle.)

this excercise made me aware of how much noticeable performance can be gained with proper tuning so although going back and forth with carbs re-adjustments can be a pain it's all worth it in the end

[Suzuki_Don]

Hey PSY, glad you got the bike back on the road and are more happy with it now. No more popping. Often when you adjust one circuit, it does affect another circuit. I had a stumble just off idle and thought it was too lean, the screws were 2.5 turns out. when I pulled the plugs they were showing a bit rich. So I turned them all in to 2 turns out and the motor runs great now, no more stumble. So what I thought was too lean was actually too rich and screwing the mixture screws in 1/2 a turn leaned the idle enough to take the stumble away. I found the beauty of checking the mixture strength at idle was to just let the bike idle for 5 mins or so with a fan on it and then check the colour of the plugs. Well that worked for me anyway.

The other thing to be mindful of is that there are three small pilot holes in the throat of the carbs adjacent to the throttle butterfly. Only one of these is controlled by the mixture screw, the other two are controlled by the size of the fuel pilot jet. So if adjusting the mixture screws does not give you the required results then you can go up or down one size on the pilots depending whether you think you are too rich or too lean where the stumbling is happening.

Hope you get it sorted soon.

[koolaid_kid]

If you have to go more than 3 turns out on the pilot screws I suggest you go up one size on the pilot jets themselves.

[psyguy]

lifting the needles by a notch took almost all the popping out!

though, something's not quite right now just off the idle... but i guess that at 4 turns out the pilot screws are too rich so i'll play again with the "highest rpm method" to set them correctly

I have set the mixture screws as per the highest rpm (at aprox 2 turns out). This has made the transition just off idle smoother but resulted (again) in some poping on deceleration - and overall on decel it felt like there's not enough gas getting through .

So I lifted the needles by one more notch. This took care of the popping and lean mixture on decel but the stumble just off the idle is even more pronaunced .


What shall I try next? I was thinking either

1. lower the needle but try larger main jets (currently I'm using the mid size of the three sizes suplied by dynajet), or
2. lower the needle and set the mixture screws out by an additional half or one turn (i.e. from aprox 2 turns to aprox 3)

What do you think?

[psyguy]

If you have to go more than 3 turns out on the pilot screws I suggest you go up one size on the pilot jets themselves.

Pilot jets are EU spec so that's already a size up from the USA spec. Not sure if this is good or bad for when tuning with the dynajet kit - they don't mention the pilot jets at all in their instructions .

[Suzuki_Don]

Psy I would try turning the mixture screws out by 1/2 a turn and see if that lessens the popping and if not happy try another 1/2 turn out (1 turn total). Hopefully this will lessen the popping and still retain the smooth transition to the needle circuit. Worth a try before pulling main jets and moving needles around again.

Good luck with it.

[GS850]

hi!

i'm also in the process of finetuning my carbs after installing k&n's but i can't seem to find a website that sells larger pilotjets for my 1979 GS850, the main jets i have.

any help? thx!

[49er]

Psy....... fit some new plugs. After setting your mixtures using the highest idle method, do some slow speed work ( in the 1/8 throttle position) and then do a plug chop. This will tell you whether the idle circuit is too lean or not. The needle will have little influence at this throttle position. Once you've established what's happening in the pilot circuit, you will be in a better position to decide whether to increase the pilot jet size or not, and then start sorting the needle position.

Popping on decel is caused by unburnt gases igniting in the exhaust. It can be an over rich indicator or the result of air leaks at the header flanges. It's more common when using certain 4-1 exhausts as the scavenging at overlap is increased along with the tendency for back flow in the intake tracts at lower rpms.

You haven't got the stock velocity stacks still fitted inside your K&N's, have you? The pods wont like that combo, as others have discovered on site.

[psyguy]

Nightmare.

I lowered the needle (back) to third notch from bottom and adjusted the mixture screws to 3 turns out (this is approx 1 more turn out from the setting got by the highest rpm method).
Starts, idles and takes off beautifully to approx half trottle. Smooth like a baby's bum.
At 3/4 throttle or WOT - it rans like CRAP! It just doesn't pull...

Main jet is DJ165 - the largest of the three supplied in the DJ kit.

Do you think this indicates lean or rich condition at WOT?


Plug chops give inconclusive results, not dramatically lean or rich (or I don't know how to read them... ). I haven't done the chop at WOT however.


Any thoughts?


I'm thinking I'll get some size 40 pilots (in place of 42.5) and lift the needle again and see what that does. I know from previous setting that just lifting the needle causes stumbles at up to 1/4 throttle, but it runs like a champ after that.